Conventionally, in the field of printed wiring board fabrication, for protecting the circuits and lowering the contact resistance and the like, the circuits are processed by metal plating. Further, as portable electronic appliances are spreading, mount parts used in the form of a chip scale package (hereinafter, referred to simply as “CSP”) or ball grid array (hereinafter, referred to simply as “BGA”) advantageous to the reduction in size are rapidly increasing. In such mount parts, a resist is formed on the entire surface of the circuit conductor on the wiring board having circuits, excluding mount pads (solder pads) and the like, and the exposed pad portions and the like are processed by metal plating. Then, the mount parts and the pads processed by metal plating are joined together by a solder ball, and the wiring board having circuits is connected to another wiring board having circuits or another part. In the metal plating processing, for securing excellent metallic bond, gold plating is usually used. Gold plating is expensive and such soft that the Mohs hardness is 2.5.
In accordance with a trend towards reduction in size and increase in density of printed wiring boards as well as application of the metal plating technique to the surface mount of electronic parts, the mainstream of the metal plating is being changed from electrolytic plating to electroless plating for the reason that the electroless plating needs no lead wire for electrodes and offers a uniform plating thickness and a flat surface and the like, and this change is rapidly progressing especially in substrates for use in portable electronic appliances. However, we have found that the printed wiring board formed by an electroless plating method has a problem in that it has poor mechanical strength due to phosphorus contained in the Ni plating layer and has poor yield of the jointing and hence, it has low solder ball connection reliability, as compared to the wiring board formed by the conventional electrolytic plating. In addition, we have found problems not only in that the solder ball connection reliability is low, but also in that the steps are increased to cause the process to be complicated.
Further, recently, in the substrate for use in portable electronic appliances, a problem arises that the substrate suffers bending or deflection due to drop impact or force caused by pressing an input key and thus mount parts, such as CSP and BGA and the like, are easily removed from the substrate surface. In order to solve this problem, a method using a substrate material which absorbs drop impact and the like and an improvement of the electroless plating method are considered, but an effective method has not yet been found.
It is an object to provide an improved process for producing a printed wiring board free of the above problems, which process is advantageous not only in that surface mount of electronic parts for achieving a reduction in size and an increase in density of the wiring board is realized using novel step procedures for the process and the steps are simplified, but also in that the steps have high yield and the printed wiring board obtained has excellent connection reliability of mount parts, as well as a photosensitive resin composition and a layer thereof having excellent plating resistance and excellent stripping properties, which are advantageously used in such a production process.